A refrigerating machine has a refrigerant circulation system including a compressor, a condenser, an expansion mechanism, an evaporator and the like. The refrigerant circulation system utilizes the phenomenon of a liquid taking heat from the surrounding at the time of vaporization, and in the system, a cycle including the compression and the temperature rise of a refrigerant vaporized in the compressor, the liquefaction of the refrigerant by heat dissipation condensation in the condenser, reduced pressure expansion in the expansion mechanism and the vaporization of the refrigerant in the evaporator is repeated. A lubricating oil for lubricating the internal slide member (called a “refrigerating machine oil”) is applied to the compressor.
In the refrigerant circulation system as described above, a part of a refrigerating machine oil is discharged from the compressor with the refrigerant due to the mechanism thereof. Therefore, compatibility with a refrigerant in addition to lubricity in the compressor (namely, lubricity in the presence of the refrigerant) is required from a refrigerating machine oil. When the refrigerating machine oil has compatibility, the refrigerating machine oil can be circulated in a refrigerant circulation system with the refrigerant and return to the compressor, and the amount of the refrigerating machine oil in the compressor can be secured enough.
However, it is not necessarily easy to achieve both the lubricity and the compatibility with a refrigerant of a refrigerating machine oil. For example, in the case of a refrigerating machine oil that exhibits high compatibility with a refrigerant, since a refrigerant is dissolved in the refrigerating machine oil, the viscosity of the whole mixture of the refrigerant and the refrigerating machine oil (working fluid composition for a refrigerating machine) decreases, and consequently, the lubricity may become insufficient. The lubricity and the compatibility with refrigerants of refrigerating machine oils vary greatly with the combination of the refrigerating machine oils and the refrigerants. Additionally, the solution of the above-mentioned subject becomes difficult, and it is also partly because the estimate of the adaptability of a refrigerant and a refrigerating machine oil is difficult.
Then, refrigerating machine oils and working fluid composition for a refrigerating machines for achieving both the lubricity and the compatibility with refrigerants have been examined. For example, in Patent Literature 1, a refrigerating machine oil composition that has compatibility suitable for hydro-fluoroalkane-based refrigerants such as 1,1,1,2-tetrafluoroethane (R134a) and the characteristics of which such as seizure resistance are improved at the same time are disclosed.
In Patent Literature 2, a working fluid composition for a refrigerating machine that is capable of maintaining a thick oil slick, has a great effect of abrasion resistance and is excellent in long-term reliability in the presence of a difluoromethane (R32) refrigerant is disclosed.